| Current technical trends in microwave dielectric ceramics applied in microwave communication are becoming research hotspots.The wide-ranging application of ceramics featuring temperature-stability and medium-permittivity as resonators and filter has led to the extensive research in this area.As a dielectric resonator/filter,three dominant properties of ceramic exist in the usage process:the permittivity（20≤εr≤70）,the temperature coefficient of resonant frequency,τf～±3 ppm/°C,and the quality factor,Q>30 000 at1 GHz.High Q×f value andεrand low dielectric loss are required to further minimize the size of the microwave devices.At present,Sr2TiO4ceramics has been extensively studied.With excellent dielectric performance,which are potential candidates for microwave dielectric ceramics.The highτfvalue is still the limit that must be optimized.In this paper,based on Sr2TiO4,the modification of Sr2TiO4ceramics was studied by substitution of Sr（A site）and Ti（B site）.At first,(Sr1-xLax)2TiO4+xceramics were synthesized by the valence balance method of electronic compensation through the La3+ion substitution in A site.When the substituting amount was small,the dielectric properties of ceramics were obviously improved,with the decreasing dielectric constant and temperature coefficient of resonant frequency,and the increasing quality factor.When x=0.01 and sintering temperature was about 1550 ℃,the ceramic with the best properties could be obtained,with the figure ofεr=36.1,Q×f=75240 GHz,τf=+116.2 ppm/℃.Then,the single-phase(Sr1-3x/2Lax)2TiO4ceramics were prepared by the valence balance method of vacancy compensation through the La3+ion substitution in A site without the effect of internal defects and the second phase.When the substituting amount x increased to 0.02,it reached the upper limit of solid solution.In the absence of the second phase,with the increase of substituting amount,the reduction of molecular molar volume and ionic polarizability led to the decrease of dielectric constant.The increase of material packing fraction caused the quality factor to increase slowly.The decrease of tolerance factor resulted in the gradual decreasing of temperature coefficient of resonant frequency with the increase of x content.However,the appearance of the second phase La2O3after reaching the upper limit of solid solution brought about the deterioration of microwave dielectric properties.When x=0.015 and sintering temperature was about 1600 ℃,the ceramic with the best properties could be obtained,with the figure ofεr=34.07,Q×f=86450 GHz,τf=+91.8 ppm/℃.Finally,Sr1.955La0.03TiO4with the perfect Q×f value was used as ceramic matrix,and Sr1.955La0.03Ti1-yCeyO4ceramics were synthesized by Ce4+ion equivalent substitution at B position.Two phases of Sr1.955La0.03Ce O4and Sr1.955La0.03TiO4existed in the ceramic early.When the substituting content increased,the dielectric constant and temperature coefficient of resonant frequency of the material decreased with the increase of substituting content,while its quality factor increased with the increase of substituting content.When y=0.7,the dielectric microwave dielectric ceramics with a temperature coefficient of resonant frequency of nearly zero were obtained whose property figure ofεr=20.18,Q×f=87651GHz andτf=-0.2 ppm/℃. |
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